Continuing interest in optical whispering gallery mode (WGM) microresonators can be attributed to their outstanding light confinement properties in applications ranging from atomic physics to optical communication systems. In general, WGMs are optical resonances created within circular structures where the optical signal travels around the circumference of the structure, undergoing repeated internal reflections at near-grazing incidence. The leakage of light can be very small in these structures, leading to high intrinsic quality factors (Q factors). The Q factor is generally defined as a measure of energy loss relative to the energy stored in a resonator (or any type of oscillating device), characterized by the center frequency of a resonance divided by its bandwidth. A “high Q” resonator is therefore associated with a relatively narrow and sharp-peaked resonance feature. These WGM microresonators typically take the form of disks, spheroids or toroids, and have an exceptionally high Q-factor as a result of the strong localization of the circulating signal.
It has previously been shown that WGMs can be excited in a silica microsphere by the evanescent coupling of light from a narrow, tapered fiber (defined as a “sensor” fiber) that is placed in contact with the microsphere. Similarly, WGMs can be excited in a second (“target”) fiber by the same contact method with a sensor fiber. Since the round-trip phase change must be an integer multiple of 2π, WGMs only exist at discrete wavelengths as determined by the diameter of the target fiber. The local diameter of a target optical fiber can therefore be deduced from the sensor's transmission spectrum, in which the wavelengths of the target fiber's WGMs appear as coupling resonances (dips) in an output spectrum. The sharpness of the resonance allows for a high resolution measurement to be made.
A prior technique of using WGMs to monitor radius variation in optical fibers required the sensor fiber to be slid along the target fiber. The physical act of moving one fiber along another was found to create problems, such as the collection of microparticles by the sensor fiber, that altered the transmission power and thus corrupted the measurement. The microparticles were also found to scratch the surface of the target fiber. A certain amount of “stick-slip” friction was also encountered.
Thus, there exists a need for an improved technique of characterizing local variations in optical fiber radius utilizing WGM monitoring without introducing the errors and corruption in results associated with the prior art method of sliding the sensor fiber along the target fiber.